Persistence of labile organic matter and microbial biomass in Antarctic shelf sediments: evidence of a sediment ‘food bank’

Abstract

The West Antarctic Peninsula (WAP) shelf experiences a highly seasonal flux of partic- ulate organic carbon following the retreat of winter sea ice, which results in deposition of labile food material for benthic detritivores. We conducted a seasonal time-series study (November 1999 through March 2001) on the WAP shelf to determine the timing and magnitude of bloom deposition on the sea floor, and the effects of seasonal deposition on microbial biomass, and persistence of labile organic material in sediments. Despite substantial seasonal changes in particle flux, the concentrations and inventories of chlorophyll a (chl a) and enzymatically hydrolyzable amino acids (EHAA) in near-sur- face sediments showed only modest seasonal variation. Interannual differences in particle flux were also large, but evidence of an elevated flux was not observed below the top 3 cm of sediment. Chl a and EHAA penetrated deeply (>5 cm) into the sediments, and had long half-lives (months to years) even in high-flux summer months. These results provide evidence of a long-term sediment 'food bank' for detritivores. Microbial biomass was relatively high in the top 2 cm of the sediments (~1.5 to 12.4 µg ATP g -1 sediment), and showed only limited response to phytodetrital deposition. Despite high sediment inventories of labile organic matter and microbial biomass, rates of organic matter mineralization were in balance with sinking particle flux. We postulate that a requirement for high substrate concentrations for microbial mineralization of organic matter at low Antarctic temperatures may promote the existence of a 'food bank' of labile material in WAP shelf sediments. Further, effects of continued climatic warming on these sediments could enhance microbial remineralization of organic matter, decreasing background levels of labile material in sediments, and altering organic matter storage and food availability for detritivores on the WAP shelf.